Simplify logic for determining what tests should run

[hellais]

We recently noticed that echcheck wasn't running because the Factory initializer didn't have enabledByDefault initialized.

In general it seems like we have multiple places that can affect wether a test may run or not:

• flags inside of the engine
• list of tests in a group in engine
• list of tests in a group in apps
• backend related feature flags

On top of this, it seems like we also apply some caching to some of these, making it harder to understand why something is not running and what the cause is.

Ideally we would consolidate this into a single source of truth and simplify all the engine related code.

In the future the source of truth will be the OONI Run link descriptors and we probably only care to selectively disable tests from the backend.